Project Summary Mucosal barriers and innate immune responses protect us from invasive infection by many non-pathogenic bacteria. However, certain bacterial pathogens have evolved strategies to cross mucosal barriers and consequently establish severe, life-threatening systemic infections. Our studies have revealed a weak link in the innate immune response that we hypothesize is exploited by these pathogens during their establishment of systemic disease. These recent studies implicate natural killer (NK) cells as a major source of interleukin (IL)- 10 production during systemic infection by the pathogen Listeria monocytogenes. IL-10 is a cytokine that many pathogenic bacteria exploit during the establishment of severe infection. It was not previously appreciated that NK cells are the major source of ?pro-bacterial? IL-10 production. We further found that a secreted L. monocytogenes bacterial virulence protein, p60, promotes NK cell IL-10 production by stimulating dendritic cells (DC) to produce IL-18 and other essential factors. Our studies here address mechanisms by which these factors coordinate NK cell activation to produce IL-10. We also investigate the impact of NK cell IL-10 on the ability of these pathogens to cross different mucosal barriers to establish systemic disease. The mechanistic information obtained through these studies may reveal strategies to promote or inhibit NK cell IL-10 production to improve human health.